Mass spectrometry is a widely accepted analytical technique for the accurate determination of molecular weights, the identification of chemical structures, the determination of the composition of mixtures and quantitative elemental analysis. It can accurately determine the molecular weights of organic molecules and determine the structure of the organic molecules based on the fragmentation pattern of the ions formed when the molecule is ionized.
Mass spectrometry relies on the production of ionized fragments from a material sample and subsequent quantification of the fragments based on mass and charge. Typically, positive, or negative ions are produced from the sample and accelerated to form an ion beam. Differing mass fractions within the beam are then selected using a mass analyzer, such as single-focusing or double-focusing magnetic mass analyzer, a time-of-flight mass analyzer, a quadrupole mass analyzer, or the like. A spectrum of fragments having different masses can then be produced, and the compound(s) within the material sample identified based on the spectrum.
Recent developments in matrix-assisted laser desorption make possible ionization of biomolecules in the 100,000 molecular weight range (Karas et al., Biomed. Environ. Mass Spectrom. 1989, 18, 841-843; Overberg et al., Rapid Commun. Mass Spectrom. 1990, 4, 293-296; Beavis et al., Rapid Commun. Mass Spectrom. 1989, 3, 233-237; and Nelson et al., Rapid Commun. Mass Spectrom. 1990, 4, 348-351). Sample molecules are mixed with a much higher proportion of relatively volatile matrix molecules. Vaporization of the matrix by the laser entrains the sample molecules into the gas phase. One matrix, frozen water (i.e., ice), has been successfully used for desorbing both proteins and nucleic acids into the gas phase (Nelson et al., Rapid Commun. Mass Spectrom. 1990, 4, 348-351). DNA molecules in the million molecular weight range have been vaporized intact with this technique (Nelson et al., Science 1989, 246, 1585-1587). In these experiments, solid samples are introduced into the vacuum chamber using a direct probe or other batch means. For example, Becker, U.S. Pat. No. 4,920,264, issued Apr. 24, 1990 , describes a method for preparing non-volatile samples for mass analysis. In this process, certain organic solvents are first added to an aqueous sample solution before the solution is frozen to form a solid matrix. Thereafter, the matrix (approximately 1 .mu.1 in volume) is placed inside the vacuum chamber of a mass spectral analysis device where the matrix is subject to desorption and ionization. Prior to the desorption step, the matrix inside the chamber is cooled to a sufficiently low temperature to prevent it from evaporating. This preparation technique is rather cumbersome, in part, because the vacuum must be broken and reestablished for each successive sample.
In an attempt to combine liquid introduction with mass spectrometry, it has been demonstrated that water introduced into a vacuum chamber through a capillary can be made to produce ice at the capillary tip (Tsuda et al., J. Chrom. 1988, 456, 363-369). As water evaporates in the vacuum chamber, energy corresponding to the heat of vaporization is removed from the end of the capillary resulting in ice formation. Ice can be made to continuously flow from the capillary by applying localized heat near the tip. This is done by passing current through a resistive wire in contact with the capillary a few millimeters from the end. However, ice formation is irregular and difficult to control so that the desired continuous introduction is difficult to achieve. Typically, a block of ice forms at the end of the capillary which prevents further sample from entering the vacuum chamber. As a corollary, combining laser desorption and ionization with continuous sample introduction, for example to measure mass spectra of effluent from separation techniques (e.g., high performance liquid chromatography, gel permeation chromatography, or capillary electrophoresis) remains a problem.